In motor-driven machine tool units such as spindles, in particular, motor spindles, rotary tables or multi-axis turning heads etc., there are in principle two different bearing arrangements as to how the rotor unit is mounted in/on the stator unit by way of its rotor shaft. The detail thereof can vary somewhat, however, depending on the embodiment.
In a first bearing principle, each case one or two angular ball bearings are usually positioned both on the front and on the rear of the rotor shaft. The angular ball bearings, arranged, for example, in what is known as the O- or X-arrangement, absorb radial forces and axial compressive forces. The rear bearings absorb radial forces and axial tensile forces.
The second possibility or bearing arrangement is referred to as the “fixed/floating bearing combination”. In this case, the bearings are usually designed such that the front bearing position or bearing unit is in the form of a fixed bearing and absorbs radial forces and axial forces in both directions. The rear bearing position or bearing unit is in this case configured as a floating bearing and absorbs only radial forces. This “fixed/floating bearing combination” is a very clearly structured, force-distributing arrangement that results from engineering mechanics.
For example, in machine tool spindles, what is known as the floating bearing is frequently realized in the rear region of the spindle or at the opposite end of the rotor shaft from the tool receptacle or workpiece receptacle with the aid of a (single) rolling bearing or cylindrical roller bearing. This cylindrical roller bearing consists generally of an inner race, a cage with integrated rollers or rolling elements and an outer race. The roller cage is guided and held either via the inner race or via the outer race.
This arrangement, which has been used in practice for several decades, also implies that the rear bearing is incapable of absorbing axial forces, with the result that the majority of the expansion movements of the shaft or rotor unit, regardless of whether they are the result of external forces or of thermal expansions or temperature influences, act only in the rear region. This in turn has the result that, for example, in motor spindles, axial movement of several tenths of a millimeter, or up to about one millimeter, sometimes arises in the rear region of the mounted motor shaft. Such movements or changes of length that are very large in/for the machine tool region do not usually have any disruptive effect at this point, however.
The great advantage of the previous fixed/floating bearing combination in such machine tool units is that the fixed bearing, which is fitted close to the tool receptacle, for example, close to the spindle, reduces axial movements of the spindle in the front region to a minimum. This in turn has the result that the tool is exposed to only very small axial displacements. This is of enormous advantage especially in the modern machine tool industry or in modern high-performance motor spindles or the like, since very high accuracy demands are nowadays made of corresponding machine tools such as CNC machines or the like. Here, accuracy not just of hundredths of a millimeter but by all means also thousandths of a millimeter has to be maintained.
Furthermore, in recent years, increasing demands have needed to be met for machining and, in particular, for parameters such as infeed and angular speed. Thus, in the meantime, tools have also come into use, wherein, during machining, the actual thickness or dimensional accuracy is determined by means of corresponding sensors and used to control the machine tool or to adjust the tool. For example, ultrasonic sensors have come into use which are arranged on the rotor shaft or in/on the (rotating) tool. For this purpose, in recent years, use has increasingly been made of electrical rotary feedthroughs or energy transmission systems, which transmit electrical energy from the static part, or from the stator, of the machine tool units to the rotating part, or the rotor, of the machine tool units.
This is realized, for example, with transmission coils which consist substantially of two operatively connected coils (with ferrite cores) and between which a non-adjustable or defined gap is present. The dimensional accuracy of the gap is very important in order not to impair energy transmission and/or signal transmission. Therefore, these transmission coils have hitherto been arranged in the front region of the workpiece, or of the front fixed bearing, since the dimensional accuracy of the gap has hitherto only been able to be ensured here.
However, a disadvantage is that, in the front region of the tool, there is the risk that, as a result of dust, chips, cooling lubricant etc., impairment or soiling of the energy transmission unit and especially of the gap and thus, inter alfa, of the signal transmission can arise. Accordingly, errors in the control of the tool and thus machining inaccuracies can result.
Thus, the transmission unit also takes up installation space, which is very disruptive in the region of tool machining and additionally there is also a risk here of the spindle colliding with the workpiece.